Circuit arrangement for operation of one or more lamps

ABSTRACT

The invention relates to a background lighting system for a liquid crystal display, more particularly to an electronic circuit for operation of one or more discharge lamps. A DC/AC full-bridge inverter circuit generates two voltages whose AC components are phase-shifted by 180°. The discharge lamps are supplied with the sum of these two AC voltages.

The invention relates to a circuit arrangement for operating one or morelow-pressure gas discharge lamps, comprising a current converter and adriving device for the current converter.

Such a circuit arrangement for operating one or more low-pressure gasdischarge lamps is known from DE 44 36 463 A1. This particularly relatesto a circuit arrangement which is suitable for operation of compactlow-pressure gas discharge lamps whose operating voltage exceeds the ACvoltage generated by the converter and is suitable for the operation ofminiature phosphor lamps. In these circuit arrangements the principle ofresonance step-up is used not only for generating the ignition voltagenecessary for the low-pressure gas discharge lamp, but also forsupplying the operating voltage of the lamp. This implies a reactivepower flux at the operating voltage.

High voltages can also be generated by using a transformer such asdescribed in U.S. Pat. No. 6,181,079 B1. Such transformers are awkwardand heavy.

It is therefore an object of the invention to indicate a simple circuitarrangement for igniting and operating such lamps. More particularly acircuit arrangement is indicated that feeds a plurality of low-pressuregas discharge lamps in the background lighting of a liquid crystaldisplay from a voltage source.

This object is achieved in accordance with the characteristic featuresof claim 1. According to the invention a second current convertergenerates a voltage shifted by 180°.

Liquid crystal displays, also called LCDs for short, are nowadays alsoused as liquid crystal picture screens. The liquid crystal picturescreens are passive display systems i.e. they do not light up bythemselves. These picture screens are based on the principle that lighteither passes the layer of liquid crystals or not. This means that anexternal light source is necessary for producing a picture. For thispurpose an artificial light is generated in the background lightingsystem. With an increasing size of the liquid crystal picture screens,also the performance level for the background lighting system of suchpicture screens increases. Lamps of small diameter are desired for thesebackground lighting systems. Compared to other low-pressure gasdischarge lamps in lighting arrangements, low-pressure gas dischargelamps in background lighting systems of liquid crystal picture screenshave a smaller inner diameter from 2 mm to 3.5 mm and, therefore, fourto eight times higher lamp voltages. Thinner lamps for LCDs such asCeralight lamps as known from EP 1 263 021 A1 work with 300 to 400 voltsoperating voltage, and cold cathode lamps in the following called ColdCathode Fluorescent Lamps or CCFLs for short, work with 600 to 800 voltsoperating voltage. The ignition voltages to start these lamps aremoreover higher by a factor of two. These high ignition and operatingvoltages for thin low-pressure gas discharge lamps are generated withouta transformer in that the low-pressure gas discharge lamps are suppliedwith power by two series-connected AC voltages. Since the two ACvoltages have a 180° phase difference, the sum of the two AC voltages isapplied to the low-pressure gas discharge lamp. In addition, these ACvoltages are generated with moderate reactive power flux in the resonantcircuits. For this purpose, the circuit arrangement has low power lossesand thus a smaller thermal load in the closed housing of the liquidcrystal picture screen.

A circuit arrangement advantageously converts DC voltage into AC voltageand feeds one or several lamps which use a full-bridge switching circuitof power switches as a current converter and two resonant circuits perlamp, each of the resonant circuits comprising one series-connectedcoil, one series-connected capacitor and one parallel-connectedcapacitor. This circuit arrangement comprises one full-bridge currentconverter and one resonant circuit per lamp. This provides that anynumber of lamps can be operated with a single current converter. Thisconverter is thus scalable. The advantage of the full-bridge converteris that it generates a double output voltage compared to a half-bridgeconverter, without utilizing a transformer. The two half bridges workwith 180° phase distance. The ignition of the lamps and the power fluxat normal operation is controlled by the switching frequency. The inputimpedance of the resonant circuit is then always ohmic inductive to havethe power semiconductors of the full-bridge converter operate withminimum switching losses. This configuration has the advantage of alower voltage load of the parallel capacitors.

The resonant circuits can additionally be constructed in three furthercircuit arrangements. Advantageously, a second circuit arrangementconverts DC current into AC current and feeds one or more lamps whichutilize a full-bridge circuit of power switches as a current converter,two series-connected capacitors and two resonant circuits per lamp, eachof the resonant circuits comprising a series-connected coil and aparallel-connected capacitor.

A third circuit arrangement advantageously converts DC current into ACcurrent and feeds one or more lamps which utilize a full-bridgeswitching circuit comprising power switches as a current converter andone resonant circuit per lamp, which resonant circuit comprises oneseries-connected coil, one series-connected capacitor and oneparallel-connected capacitor.

A fourth circuit arrangement advantageously converts DC current into ACcurrent and feeds one or more lamps which utilize a full-bridgeswitching circuit with power switches as a current converter, twoseries-connected capacitors and one resonant circuit per lamp, whichresonant circuit comprises one series-connected coil and oneparallel-connected capacitor.

The parallel-connected capacitor is advantageously formed at leastpartly by a parasitic capacitance between the lamps and a metallicportion, thus the lamp electrodes and the electrically conductive partsof the display, for example, of the reflector.

To better understand the invention, an example of embodiment will befurther explained hereinbelow with reference to the drawing in which:

FIG. 1 shows a circuit arrangement for converting DC current into ACcurrent and for feeding one or more low-pressure gas discharge lamps,

FIG. 2 shows a timing diagram with a rectangular signal waveform,

FIG. 3 shows a timing diagram with a sine curve,

FIG. 4 shows a timing diagram with two sine curves phase-shifted by180°,

FIG. 5 shows a second circuit arrangement for converting DC current intoAC current and for feeding one or more low-pressure gas discharge lamps,

FIG. 6 shows a third circuit arrangement for converting DC current intoAC current and for feeding one or more low-pressure gas discharge lamps,

FIG. 7 shows a fourth circuit arrangement for converting DC current intoAC current and for feeding one or more low-pressure gas discharge lamps,and

FIG. 8 shows a diagram with a voltage ratio plotted against frequency.

FIG. 1 shows an electronic circuit arrangement 1 comprising afull-bridge switching circuit 2, a voltage source 3, two low-passfilters 4 and 5, a first lamp switching circuit 6, two further low-passfilters 7 and 8 and a second lamp switching circuit 9. Electricallyconducting lines 10, 11 and 12 lead to further lamp switching circuits(not shown). The full-bridge switching circuit 2 also called full-bridgeinverter in the following, comprises a control circuit 13 and twocurrent converters 14 and 15. The current converter 14, in the followingalso called inverter, includes two power switches 16 and 17, and thesecond inverter 15 also includes two power switches 18 and 19. Powersemiconductors such as bipolar transistors, IGBTs (Integrated GateBipolar Transistors) are also MOSFETs are used as power switches. Tonefirst lamp switching circuit 6 includes two series-connected coils 20and 21, two parallel-connected capacitors 22 and 23 and one low-pressuregas discharge lamp 24. The second lamp circuit 9 has a similar structurewith components 20 to 24. The control circuit 13 controls the firstinverter 14 so that the power semiconductors 16 and 17 open and close ina push-pull mode. A rectangular signal waveform evolves at a node 25between the power semiconductors 16 and 17. The control circuit 13controls the second inverter 15 so that the power semiconductors 18 and19 also open and close in a push-pull mode. A rectangular signalwaveform also evolves at a node 26 between the power semiconductors 18and 19. The two inverters 14 and 15 work in phase opposition, so thattwo rectangular signal waveforms evolve shifted by 180°. The low-passfilters 4, 5, 7 and 8 filter out the high-frequency components, so thattwo sinusoidal signals shifted in phase by 180° reach the lamps 24. Theseries-connected coil 20 and the parallel-connected capacitor 22 form afirst resonant circuit 20, 22, the coil and the capacitor 23 form asecond resonant circuit 21, 23. The low-pass filters 4 and 5, the coils20 and 21 and the lamp 24 are connected in series between the two nodes25 and 26. The capacitors 22, 23 are connected in parallel to the lamp24 and to the minus pole of the DC voltage source 3. The half lampvoltage is applied via the capacitors 22 and 23, respectively.

FIG. 2 shows a rectangular signal waveform 31 which arises at the node25. A similar signal waveform arises at node 26. The two rectangularsignal waveforms are phase-shifted by 180°.

FIG. 3 shows a sinusoidal signal waveform 32 which evolves as a resultof the smoothing by the low-pass filter 4.

FIG. 4 shows a sine curve 32 and a second sine curve 33 shifted by 180°,which is filtered by the low-pass filter 5. In this way a maximumvoltage amplitude 34 corresponding to the value of the voltage supply 3arises at the lamp 24.

FIG. 5 shows a second circuit arrangement 41 comprising a full-bridgeinverter 2 and the lamp switching circuits 6 and 9. Two low-pass filters42 and 43 filter out the high-frequency components for all the lampcircuits 6 and 9.

FIG. 6 shows a third circuit arrangement 51 comprising the full-bridgeinverter 2, the voltage source 3 and two lamp switching circuits 52 and53. Between the two nodes 25 and 26 in the lamp circuit 52 is connecteda capacitor 54, a coil 55 and a capacitor 56 which together work as alow-pass filter, and a low-pressure gas discharge lamp 24 in parallelwith capacitor 56. The coil 55 and the capacitor 56 form a resonantcircuit 55, 56.

The coil 55 has double the inductance of coil 20, the capacitor 56 halfthe capacitance of the capacitor 22. There is a voltage drop across thecapacitor 56, which drop corresponds to the lamp voltage.

FIG. 7 shows an electrical circuit arrangement 61 with twoseries-connected capacitors 62, 63 which work for all the lamp circuits52, 53.

FIG. 8 shows a diagram in which the voltage is plotted againstfrequency. The AC power gain function of a resonant circuit is shown asa function of the switching frequency. To ignite a low-pressure gasdischarge lamp, the full-bridge starts with a starting frequency 71,reduces the switching frequency until the lamp ignites at an ignitionfrequency 72 and reduces the switching frequency further to an operatingfrequency 73.

List of Reference Characters:

1 circuit arrangement

2 full-bridge inverter

3 voltage source

4 low-pass filter

5 low-pass filter

6 lamp switching circuit

7 low-pass filter

8 low-pass filter

9 lamp switching circuit

10 electrically conducting line

11 electrically conducting line

12 electronically conducting line

13 control circuit

14 inverter

15 inverter

16 power switch

17 power switch

18 power switch

19 power switch

20 series coil

21 series coil

22 capacitor

23 capacitor

24 lamp

25 node

26 node

31 rectangular signal waveform

32 sinusoidal fundamental wave

33 second sinusoidal fundamental wave

34 voltage amplitude

41 second circuit arrangement

42 low-pass filter

43 low-pass filter

51 third circuit arrangement

52 lamp switching circuit

53 lamp switching circuit

54 capacitor

55 coil

56 capacitor

61 four circuit arrangement

62 capacitor

63 capacitor

71 start frequency

72 ignition frequency

73 operating frequency

1. A circuit arrangement (1, 41, 51, 61) for operating one or morelow-pressure gas discharge lamps (24), comprising a current converter(14) and a driving device (13) for the current converter (14),characterized in that a second current converter (15) generates avoltage (32, 33) phase-shifted by 180°.
 2. A circuit arrangement (1) forconverting DC current into AC current and for feeding one or morelow-pressure gas discharge lamps (24) which utilizes a full-bridgeswitching circuit (2) with power switches (16, 17, 18, 19) as currentconverters (14, 15) and two resonant circuits (4, 5, 20, 21, 22, 23) perlamp (24), each of the resonant circuits (4, 5, 20, 21, 22, 230 having aseries-connected coil (20, 21), one series-connected capacitor (4, 5)and one parallel-connected capacitor (22, 23).
 3. A circuit arrangement(41) for converting DC current into AC current and for feeding one ormore low-pressure gas discharge lamps (24), which utilizes a full-bridgeswitching circuit (2) including power switches (16, 17, 18, 19) ascurrent converters (14, 15), two series-connected capacitors (42, 43)and two resonant circuits (20, 21, 22, 23) per lamp (24), each of theresonant circuits (20, 21, 22 or 23) having one series-connected coil(20, 21) and one parallel-connected capacitor (22, 23).
 4. A circuitarrangement (51) for converting DC current into AC current and forfeeding one or more low-pressure gas discharge lamps (24) which utilizesa full-bridge switching circuit (2) with power switches (16, 17, 18, 19)as current converters (14, 15) and one resonant circuit (54, 55, 56) perlamp (24), which resonant circuit includes one series-connected coil(55), one series-connected capacitor (54) and one parallel-connectedcapacitor (56).
 5. A circuit arrangement (61) for converting DC currentinto AC current and for feeding one or more low-pressure gas dischargelamps (24), which utilizes a full-bridge switching circuit (2) withpower switches (16, 17, 18, 19) as current converters (14, 15), twoseries-connected capacitors (62, 63) and one resonant circuit (55, 56)per lamp (24), which resonant circuit includes a series-connected coil(55) and a parallel-connected capacitor (56).
 6. A circuit arrangementas claimed in claim 2, characterized in that the parallel-connectedcapacitor (22, 23, 56) is formed at least partly by a parasiticcapacitance between the lamp (24) and a metallic part.
 7. A liquidcrystal display on which a video signal of a computer or of a televisionset can be represented, comprising a circuit arrangement (1, 41, 51, 61)as claimed in claim 1.